Sensitive moving chart graphic recorder



Aug. 5, 1952 T. A. RICH 2,606,092

SENSITIVE MOVING CHART GRAPHIC RECORDER Filed Dec. 20, 1947 2 SHEETSSHEET l 5, 1952 T. A. RlCH 2,606,092

SENSITIVE MOVING CHART GRAPHIC RECORDER Filed Dec. 20, 1947 2 SHEETS-SHEET 2 Fig.4.

IYn/entova The; QdOYQ A. my m Has Attnrm Patented Aug. 5, 195 2 SENSITIVE MOVING CHART GRAPHIC RECORDER Theodore A. Rich, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York n Application December 20, 1947, Serial No. 792,879.

1 Claim. 1

My invention relates to improvements in recorders of the type adapted to make a graphic record of a measured variable upon a moving chart, and particularly to improvements in such recorders adapted to measure and record the magnitude of a variable voltage. The objects of my invention are to provide an improved graphic recorder of high sensitivity in which errors due to the starting torque of signal-responsive members are minimized, and to provide improved apparatus for measuring and recording a variable voltage. The means by which I attain these objects are hereinafter set forth.

The features of my invention which are believed to be novel and patentable, will be pointed out in the claim appended hereto. For a better understanding of my invention, reference is made in the following description to the accompanying drawings in which Fig. 1 is an elevation of one form of my invention; Fig. 2 is a vertical section along the line 2-2, Fig. 1; Fig. 3 is a fragmentary view of a modification of the apparatus of Fig. 1; Fig. 4 is an elevation of another form of my invention; and Fig. 5 is a section along the line 5-5, Fig. 4. Like reference characters designate like parts throughout the drawings.

Referring now to Figs. 1 and 2 of the drawings, a roller supports a chart II upon which the value of a variable measured voltage, or any other value which may be represented by such a voltage, is graphically recorded as hereinafter explained. A motor l2, acting through gears I3 and I4 as shown, rotates roller I0 so that chart I I is moved past pin 24 at the desired recording speed. A synchronous or other constant speed motor I5 rotates a lead screw I6 at a constant speed. The same motor may be used for I5 and I2 by suitable gearing. A variable speed electric motor I'I, governed as hereinafter described, rotates a lead screw I8 in the opposite direction at a speed which varies responsive to any error between the voltage indicated by the recorder and the measured voltage, the differences in speed of the two screws bein proportional to such error. The two lead screws are axially parallel one to the other and to the chart, as shown. A gear I9, rotatable about an axle 20, cooperates with both lead screws simultaneously so that when the two screws rotate with equal speeds gear I9 is rotated thereby, but is not translated. If, however, screw I3 rotates faster than screw Iii, gear I9 is translated laterally to the left, and if screw I8 rotates slower than screw I6, gear I9 is translated laterally to the right, the rate of such translation being proportional to thedifierence in speed of the two screws. A carriage 2| is supported by a track 22 and connected to axle 20 as shown, so that the carriage travels laterally across the chart responsive to'the translation of gear I9. Stops 23 at each end of the track limit the travel of the carriage to the width of the chart. Motors l5 and I! are normally designed to deliver little power, so that the speeds of the motors vary responsive to small load changes developed when the travel of carriage 2| is prevented by stops 23,'and no damage to the apparatus results. If larger motors are used, stops 23 may be replaced by limit switches. A pen, pencil, or other writing member 24 is attached to the carriage and positioned on the chart. The pen moves to the left across the chart responsive to an increase in the value of the measured voltage, and moves to the right responsive to the decrease in such value.

Carriage 2| is adapted to position an adjustable tap 26 on a resistor 2'! responsive to the travel of the carriage. The voltage to be measured X is applied between two terminals 28 and 29. Terminal 29 is connected to an adjustable tap 30 on a resistor 3| which is connected in parallel with resistor 2'! as shown. The position of tap 30 determines the zero or reference position of tap 26, as hereinafter explained. A battery or other voltage source 32 connected in series with a variable resistor 33 as shown, provides a constant voltage across resistors 2! and 3|, so that a voltage is obtained between tap 28 and tap 30 which is proportional to the difference in position of the two taps, or, stated differently, to the displacement of tap 26 away from a zero or reference position determined by tap 39, and is also proportional to the total voltage across resistors 2! and 3 I. This latter voltage may be adjusted by varying the value of resistor 33. The difference between the voltage obtained between taps 2'3 and 30 and the measured voltage X is obtained between tap 26 and terminal 28, since the two voltages are opposed in a series connected circuit. This difference or error voltage is applied between the grid and cathode of a vacuum tube 34 by connections as shown. The plate and cathode of tube 34 are connected in series with a voltage source 35 and Variable speed electric motor IT, as shown, so that the plate current of vacuum tube 34 controls the speed of motor iii. In place of vacuum tube 34 and its associated circuits any'means for amplifying direct current may be used. Requirements for increased sensitivity or for larger currents to operate motor it will often make more elaborate multi-stage cir than screw it, carriage 2| is moved to the left,

and the voltage between tap 2t and terminal 23 I is reduced. In like manner, it the voltage decreases carriage 2| is moved to the right and the voltage is increased, any-unbalance voltage in either direction being quicklyandaccurately cor-.

rected by a movement of the carriage. It is thus evident that the position ofcarriage. 2!, or. .its displacement across chart H from its zero or reference position, is a measure of the voltage 21' applied between terminals 28 and 29. By adjusting resistor 33 and tap both the magnitude of the. voltageacross resistor 27 and the potential of terminal 29 relative thereto may be varied, to calibrate both thezerof position and the sensitivity of the recorder. In-Eig. l, the adjustment shown provides'ga'ze'rocenter instrument, since with zero input taps 2;o ,and 39 are substantially in line. Zero position is indicated byreference line 25 on the chart.

A cathode resistor .35. is preferably provided to supply the normal operating bias for vacuumtube 34, The normal error voltage, that is, the voltage betweenterminal 28 and tap 26 when the carriage isnot in motion, may be made zero by adjusting the value of resistor 33, and the impedance between terminals Z Band 25] will then theoretically be infinite, since any voltage applied between terminals 28 and"29 will'be'exactly balanced by the voltage between taps 28 and 30. Thus no current will flow between the terminals and voltages in a high irhpedance external circuit'can be measured without loading the external circuit. Grid-leak resistor 31 does not lower the impedance between terminals 28 and 29 since the balance voltage across such resistor is zero,and no current flows therethrough.

Having motor-H and screw* l8 rotate continuously and always inthe same direction provides several advantages for my -apparatusc motor I? need not be reversible; greatlyv increased sensitivityis obtained since there is no starting torque to be overcome; increasedaccuracy is obtained since the motor is. initially. operating within a speed range in which accurate. speed control is possible-for example, the speed of a variablespeed motor can be changed from .900 revolutions per minute to 901 revolutions per minute responsive to a small speed-controlling signal much more easily and accurately inmost cases thanthespeed of the same motor can be changedfrom 0 to 1 revolution per minute; and improved linearity can be obtained since the range of speed variations may be made relativelysinallpompared to the initial speed. Y

Thos 'skilled in the art will perceive th at;v arious changes in the arrangement and form of parts can be made withoutaltering the operating principles of my invention. 7 One such modification is shown in Fig. 3, which is a fragmentary. view showing the modified portion of the apparatus. Constant-speed motor [5 acting through gear 33 and an elongated pinion 39 as shown, rotates gear 40 at a constant speed. Variable speed motor I1 rotates lead screw [8 as hereinbefore explained. Motors I51; and I1 rotatein the same direction, so that pinion 39 and lead screw 18 rotate in opposite directions. Gear 40 is provided with a threaded axial hole and mounted upon lead screw l8, so that the threads of the hole engage the threads of the screw. Thus, when motors I511 and Il rotate with equal speeds and in the same direction, gear 40 and screw [8 likewise rotate with equalspeeds in the same direction, and gear 40 is not translated. But if the speeds of the motors are unequal, gear 40 rotates relative to screw it and is thereby translated either to the left or to the right depending upon which motor rotates with the faster speed. Carriage Zla is provided with a slot in which gear 40 is positioned as shown, so that carriage Zla is moved to the left or to theri'ghtiresponsive to like translations of gear 40, Operation of the apparatus is other- Wise identical to operation of the apparatus of Fig. I, herebefore described.

Referring now to Figs. 4 and 5, another form of recorder embodying my invention is shown, in which theprinciples of operation are the same as in the-recorder shown in Figs. 1 and2, although the form and arrangement-of certain partsaredifferent. In the following description reference is made to the descriptionhereinbefore given for similar parts in the apparatus of Figs. land 2. A chart llais supported by roller 10 which is rotated by motor l2andgearsv I3 andv M as hereinbefore explained. Two shafts 4| and t2 are axially positioned in a. straight lineas shown. Constant speed motor l5.rotates shaft at a constant speed andvariable-speed motor I"! rotates shaft 42 in the opposite direction at a speed which-varies responsive to any error between the voltage-indicated by the recorder andthe measured voltage, in the manner hereinbefore explained. A set of difierential gears tit-contained in a housing 44 interconnect the two shafts as shown. A pen arm 45 is attachedto the differential housing and extends outward therefrom substantially"perpendicular tothe common axis of the-two shafts. Thepen armis therebyadapted to rotate about the axis of the shafts, and its outer end travels in an arc laterally across the chart responsive to such rotation. Pins 46, or limit switches, limit such travel to thewidth ofthe chart. Theaction of the differential gears issuch that the rotation of the pen; arm isproportional tothe difference between the-rotations of the two shafts, Any'error betweenthe indicated voltage and themeasured voltage-causesa difierence in shaft speeds as hereinbefore explained, and a correspondingrotation of the pen arm. A pen 24, or other writing member, is attached to the outer endof penarm 45 andis moved laterally across the chart by such rotation. Resistoigl'la, corre pe d in nct on s ister; F is shaped as-asection of circular arc, as shown,; so that adjustable tap '25 may bepositioned by pen arm 45. The remainder of the electrical circuit is d tical nf rm n no r i n it h c r ui shown in Fig; 1, and so is omitted from- Figs. 4 and 5 to simplify the drawing.

In-accordance with the-patentstatutes, I;h ave described the principle of my invention and the apparatus which; I now consider to represent the best embodiment thereof, but I wish it to be understood that the apparatus shown is illustrative only n ha e v n ionmav beca r ed out b other means.

What I claim as, neweand desire to, secure by Letters Patent of the United States is:

Apparatus to graphicallyrecord upon amoving chart the value of .a variablevoltage, comprising means to support and move said chart, two shafts positioned to have their axes substantially in a straight line, a constant speed electric motor connected to rotate one shaft in one direction at a constant speed, a variable-speed electric motor connected to rotate the other shaft in the opposite direction at a variable speed, speed control means for the variable speed motor operable in response to a voltage to be recorded, a pen arm which rotates about the common axis of the shafts and extends outward substantially perpendicular thereto, the outer end of said pen arm traveling laterally across the chart responsive to such rotation, means including a differential gear to rotate the pen arm an amount proportional to the difference between the rotations of the two shafts, and a writing member attached to the outer end of the pen arm and positioned th reby on the chart.

THEODORE A. RICH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

